The present invention relates generally to semiconductor devices, and more particularly to power MOSFET devices.
Power MOSFET devices are employed in applications such as automobile electrical systems, power supplies, and power management applications. Such devices should sustain high voltage in the off-state while having a low voltage drop and high current flow in the on-state.
FIG. 1 illustrates a typical structure for an N-channel power MOSFET. An N-epitaxial silicon layer 1 formed over an N+ silicon substrate 2 contains p-body regions 5a and 6a, and N+ source regions 7 and 8 for two MOSFET cells in the device. P-body regions 5 and 6 may also include deep p-body regions 5b and 6b. A source-body electrode 12 extends across certain surface portions of epitaxial layer 1 to contact the source and body regions. The N-type drain for both cells is formed by the portion of N-epitaxial layer 1 extending to the upper semiconductor surface in FIG. 1. A drain electrode is provided at the bottom of N+ substrate 2. An insulated gate electrode 18 typically of polysilicon lies primarily over the body and portions of the drain of the device, separated from the body and drain by a thin layer of dielectric, often silicon dioxide. A channel is formed between the source and drain at the surface of the body region when the appropriate positive voltage is applied to the gate with respect to the source and body electrode.
The on-resistance of the conventional MOSFET shown in FIG. 1 is determined largely by the drift zone resistance in epitaxial layer 1. The drift zone resistance is in turn determined by the doping and the layer thickness of epitaxial layer 1. However, to increase the breakdown voltage of the device, the doping concentration of epitaxial layer 1 must be reduced while the layer thickness is increased. Curve 20 in FIG. 2 shows the on-resistance per unit area as a function of the breakdown voltage for a conventional MOSFET. Unfortunately, as curve 20 shows, the on-resistance of the device increases rapidly as its breakdown voltage increases. This rapid increase in resistance presents a problem when the MOSFET is to be operated at higher voltages, particularly at voltages greater than a few hundred volts.
FIG. 3 shows a MOSFET that is designed to operate at higher voltages with a reduced on-resistance. This MOSFET is disclosed in paper No. 26.2 in the Proceedings of the IEDM, 1998, p. 683. This MOSFET is similar to the conventional MOSFET shown in FIG. 2 except that it includes p-type doped regions 40 and 42 which extend from beneath the body regions 5 and 6 into the drift region of the device. The p-type doped regions 40 and 42 define columns in the drift region that are separated by n-type doped columns, which are defined by the portions of the epitaxial layer 1 adjacent the p-doped regions 40 and 42. The alternating columns of opposite doping type cause the reverse voltage to be built up not only in the vertical direction, as in a conventional MOSFET, but in the horizontal direction as well. As a result, this device can achieve the same reverse voltage as in the conventional device with a reduced layer thickness of epitaxial layer 1 and with increased doping concentration in the drift zone. Curve 25 in FIG. 2 shows the on-resistance per unit area as a function of the breakdown voltage of the MOSFET shown in FIG. 3. Clearly, at higher operating voltages, the on-resistance of this device is substantially reduced relative to the device shown in FIG. 1, essentially increasing linearly with the breakdown voltage.
The improved operating characteristics of the device shown in FIG. 3 are based on charge compensation in the drift region of the transistor. That is, the doping in the drift region is substantially increased, e.g., by an order of magnitude or more, and the additional charge is counterbalanced by the addition of columns of opposite doping type. The blocking voltage of the transistor thus remains unaltered. The charge compensating columns do not contribute to the current conduction when the device is in its on state. These desirable properties of the transistor depend critically on the degree of charge compensation that is achieved between adjacent columns of opposite doping type. Unfortunately, nonuniformities in the dopant gradient of the columns can be difficult to avoid as a result of limitations in the control of process parameters during their fabrication. For example, diffusion across the interface between the columns and the substrate and the interface between the columns and the p-body region will give rise to changes in the dopant concentration of the portions of the columns near those interfaces.
The structure shown in FIG. 3 can be fabricated with a process sequence that includes multiple epitaxial deposition steps, each followed by the introduction of the appropriate dopant. Unfortunately, epitaxial deposition steps are expensive to perform and thus this structure is expensive to manufacture. Another technique for fabricating these devices is shown in copending U.S. application Ser. No. 09/970,972, in which a trench is successively etched to different depths. A dopant material is implanted and diffused through the bottom of the trench after each etching step to form a series of doped regions (so-called xe2x80x9cfloating islandsxe2x80x9d) that collectively function like the p-type doped regions 40 and 42 seen in FIG. 3. However, the on-resistance of a device that uses the floating island technique is not as low as an identical device that uses continuous columns.
Accordingly, it would be desirable to provide a method of fabricating the MOSFET structure shown in FIG. 3 that requires a minimum number of deposition steps so that it can be produced less expensively while also allowing sufficient control of process parameters so that a high degree of charge compensation can be achieved in adjacent columns of opposite doping type in the drift region of the device.
The present invention provides a high voltage semiconductor device and a method of forming same. The method proceeds by:
A. providing a substrate of a first or second conductivity type;
B. forming a voltage sustaining region on the substrate by:
1. depositing an epitaxial layer on the substrate, the epitaxial layer having a first conductivity type;
2. forming at least one trench in the epitaxial layer;
3. depositing a barrier material along the walls of the trench;
4. implanting a dopant of a second conductivity type through the barrier material into a portion of the epitaxial layer adjacent to and beneath the bottom of the trench;
5. diffusing the dopant to form a first doped layer in the epitaxial layer;
6. removing the barrier material from at least the bottom of the trench;
7. etching the trench through the first doped layer to a greater depth and repeating steps (B.3)-(B.5) to form a second doped layer vertically below the first doped layer;
8. removing the barrier material from surfaces of the trench.
9. depositing a diffusion facilitating material along the walls of the trench, the implanted dopant having a higher diffusion coefficient in the deposited material than in the epitaxial layer of the voltage sustaining layer;
10. diffusing the dopant into the diffusion facilitating material so that the dopant diffuses into sidewalls of the trench between the first and second doped layers;
11. depositing a filler material in the trench to substantially fill the trench; and
C. forming above but in contact with the voltage sustaining region at least one region of the second conductivity type.
In accordance with one aspect of the invention the method also includes the steps of: forming a gate conductor above a gate dielectric region; forming at least one body region in the epitaxial layer to define a drift region therebetween, the body region having a second conductivity type; and forming at least one source region of the first conductivity type in the at least one body region.
In accordance with another aspect of the invention, the barrier material is an oxide material.
In accordance with another aspect of the invention the oxide material is silicon dioxide.
In accordance with another aspect of the invention, the material filling the trench is a dielectric material such as silicon dioxide, silicon nitride or high resistivity polycrystalline silicon.